Steel material excellent in resistance of ductile crack initiation from welded heat affected zone and base material and method for manufacturing the same

ABSTRACT

A steel material has a composition of C: 0.02 to 0.2%, Si: 0.01 to 0.5%, Mn: 0.5 to 2.5%, P: 0.05% or lower, S: 0.05% or lower, Al: 0.1% or lower, and N: 0.01% or lower and, as required, one or two or more elements selected from Cu: 0.01 to 2%, Ni: 0.01 to 5%, Cr: 0.01 to 3%, Mo: 0.01 to 2%, Nb: 0.1% or lower, V: 0.1% or lower, Ti: 0.1% or lower, B: 0.01% or lower, Ca: 0.01% or lower, and REM: 0.1% or lower in terms of % by mass, and the balance Fe with inevitable impurities, in which the microstructure at the ¼ position of the plate thickness contains ferrite and a hard phase, the area fraction of the hard phase is 50 to 90%, and the average aspect ratio of the ferrite is 1.5 or more.

RELATED APPLICATIONS

This is a §371 of International Application No. PCT/JP2009/071908, withan international filing date of Dec. 25, 2009 (WO 2010/074347 A1,published Jul. 1, 2010), which is based on Japanese Patent ApplicationNos. 2008-333204, filed Dec. 26, 2008, and 2008-333205, filed Dec. 26,2008, the subject matter of which is incorporated by reference.

TECHNICAL FIELD

This disclosure relates to steel materials suitable for use in weldedstructures such as pipelines, bridges, and architectural structures,requiring structural safety and a method for manufacturing the same andparticularly relates to one excellent in resistance of ductile crackinitiation from welded heat affected zone and a base material.Specifically, the disclosure is targeted to steel materials forstructures having excellent resistance of ductile crack initiation fromwelded heat affected zone and a base material and having strength ofTensile strength: 490 MPa or more in TS and high toughness ofDuctile-brittle fracture transition temperature of Charpy impact test(according to the regulation of JIS Z 2242): vTrs of 0° C. or lower.

BACKGROUND

It is known that when the welded structures such as pipelines, bridges,and buildings, are exposed to large external force of an earthquake orthe like, ductile crack initiates in a stress concentration zone, suchas a weld toe, and the generated ductile crack serves as a trigger tocause brittle fracture, resulting in break and fracture of thestructures in some cases.

It is important that steel materials constituting the same are excellentin resistance of ductile crack initiation to avoid such break andfracture of the welded structures.

Japanese Unexamined Patent Application Publication No. 2008-202119discloses a high tensile-strength steel material excellent in resistanceof ductile crack initiation in which, in the microstructure a steelmaterial surface zone, the ferrite area fraction is 10 to 40%, thebainite area fraction is 50% or more, and the average grain size is 5 μmor lower.

Japanese Unexamined Patent Application Publication No. 2000-3281777discloses a steel plate excellent in arrestrability and resistance ofductile fracture in which the microstructure is substantiallyconstituted by a ferrite structure, a pearlite structure, and a bainitestructure and, when divided into three layers of both surface zones andthe central zone in the plate thickness direction of the steel plate,each zone has a specific microstructure.

Both the surface zones of the steel plate are constituted by a layerwhich has 50% or more of a ferrite structure containing ferrite grainsin which the equivalent (circle) diameter is 7 μm or lower and theaspect ratio is 2 to 4 over 5% or more of the plate thickness of each ofthe structure zones and in which the bainite area fraction of theportion is 5 to 25% or lower. The central zone in the plate thicknessdirection of the steel plate is constituted by a layer which containsferrite grains in which the equivalent (circle) diameter is 4 to 10 μmand the aspect ratio is 2 or lower over 50% or more of the platethickness and in which the bainite area fraction of the zones is 10% orlower.

More specifically, the technique of JP '177 is directed to a steel platein which three layers having a ferrite/pearlite structure containingferrite grains different in the aspect ratio are present in the platethickness direction from the plate surface of the steel plate andfurther in which a bainite structure which is a hard phase isappropriately dispersed in a soft phase which is the ferrite/pearlitestructure. The technique increases the arrestrability by positivelyforming processed ferrite grains having a high aspect ratio and alsoappropriately dispersing a bainite structure on each of both the surfacezones of the three zones of the steel plate and, in contrast, increasesextension characteristics, which are important to ductile fracture atroom temperature, by controlling the central zone of the steel plate tohave a uniform equiaxed ferrite grain structure and also suppressing abainite structure, and thus satisfies both opposite characteristics of“arrestrability” and “ductile fracture characteristics” by controllingboth the surface zones and the central zone of the steel plate to thethree-layer structure.

Also the technique of Japanese Unexamined Patent Application PublicationNo. 2003-221619 is directed to a technique of obtaining deformed ferritegrains on the steel plate surface zone of ferrite/pearlite steel andalso controlling the microstructure of the central zone to a uniformequiaxed ferrite grain structure similarly as the technique of JP '177.

More specifically, JP '619 discloses a method for manufacturing a thicksteel plate excellent in arrestrability and ductile fracturecharacteristics in which the rolling conditions are strictly controlledso that the steel plate surface zone has a specific microstructure.

Specifically, when the thickness during plate rolling is defined as t,an equivalent plastic strain ε of ε≧0.5 in a non-recrystallizationtemperature zone of Ar₃ transformation point or more and 900° C. orlower is given to a surface layer zone of 0.05 t or more and 0.15 t orlower from both the surfaces in the plate thickness direction.

Thereafter, the surface layer zone is cooled to a temperature range of450 to 650° C. at a cooling rate of 2 to 15° C./s while maintaining thetemperature of the central zone defined as t/4 to 3t/4 of the platethickness at the Ar₃ transformation point or more within a period oftime when the residual and cumulative equivalent plastic strain εr ofthe surface layer zone satisfies εr≧0.5, and subsequently rolling isrestarted.

In the restarted rolling, the residual and cumulative equivalent plasticstrain εr of 0.35≦εr<0.55 is given to the central zone to complete therolling at the Ar₃ transformation point or more and also the surfacelayer is recuperated to the Ar₃ transformation point or lower byprocessing heat and internal sensible heat, and thereafter cooling isperformed in such a manner that the average cooling rate is 1 to 10°C./s.

The techniques of JP '119, JP '177 and JP '619 all relate to techniquesof forming fine subgrains in austenite to miniaturize the structureafter transformation by performing rolling in a non recrystallizationzone (fine grain temperature zone) of austenite or performing rolling ata rolling finish temperature Ar₃ or more.

However, according to the techniques of JP '119, JP '177 and JP '619,when the surface layer structure changes to the welded heat affectedzone by welding or the like, there is a concern that the effect ofresistance of ductile crack initiation is lost.

Moreover, in all of a scale breaker for use in treatment of the surfaceof a slab extracted from a heating furnace described in Examples of JP'119, two-stage rolling of rolling in a pulverization temperature rangeand rolling in a set temperature zone described in Examples of JP '177,and various kinds of rolling or temperature control for separatelycreating the structure of a surface layer and the structure inside asteel plate described in JP '619, the manufacturing process iscomplicated.

Then, in view of the problems of such former techniques, it could behelpful to provide steel materials excellent in resistance of ductilecrack initiation from the welded heat affected zone and a base materialby a simple method and a method for manufacturing the same.

SUMMARY

We conducted extensive research on the microstructure of base materialexcellent in resistance of ductile crack initiation of welded heataffected zones and found that, when the microstructure of a basematerial has a ferrite and a hard phase in which the average aspectratio of the ferrite and the area fraction of the hard phase are at the¼ position of the plate thickness exhibiting an average structure in theplate thickness direction of a steel plate, the resistance of ductilecrack initiation is excellent also in the welded heat affected zone andsuch a steel material is excellent also in the resistance of ductilecrack initiation of the base material, and further manufacturingconditions of a steel plate having the microstructure.

We thus provide:

-   -   (1) A steel material excellent in resistance of ductile crack        initiation from welded heat affected zone and a base material        has a composition of C: 0.02 to 0.2%, Si: 0.01 to 0.5%, Mn: 0.5        to 2.5%, P: 0.05% or lower, S: 0.05% or lower, Al: 0.1% or        lower, and N: 0.01% or lower in terms of % by mass, and the        balance Fe with inevitable impurities, in which the        microstructure at the ¼ position of the plate thickness contains        ferrite and a hard phase, the area fraction of the hard phase is        50 to 90%, and the average aspect ratio of the ferrite is 1.5 or        more.    -   (2) The steel material excellent in resistance of ductile crack        initiation from welded heat affected zone and a base material        according to (1), further contains, in the chemical composition,        one or two or more elements selected from Cu: 0.01 to 2%, Ni:        0.01 to 5%, Cr: 0.01 to 3%, Mo: 0.01 to 2%, Nb: 0.1% or lower,        V: 0.1% or lower, Ti: 0.1% or lower, B: 0.01% or lower, Ca:        0.01% or lower, and REM: 0.1% or lower in terms of % by mass.    -   (3) In the steel material excellent in resistance of ductile        crack initiation from welded heat affected zone and a base        material according to (1) or (2) above, the structure on the        surface of a steel plate contains ferrite and a hard phase, the        area fraction of the ferrite exceeds 40%, and the average aspect        ratio of the ferrite grain size exceeds 2.    -   (4) A method for manufacturing a steel material excellent in        resistance of ductile crack initiation from welded heat affected        zone and a base material includes reheating a steel base        material having the chemical compositions of (1) or (2) to        1000° C. or more, rolling the same in such a manner that the        rolling reduction rate in a temperature range of 900° C. or more        is 50% or more and the rolling finish temperature is Ar₃ point        to Ar₃-50° C., starting water cooling at Ar₃-10° C. to Ar₃-70°        C., and terminating the water cooling at 500° C. or lower.    -   (5) The method for manufacturing a steel material excellent in        resistance of ductile crack initiation from welded heat affected        zone and a base material according to (4) further includes,        after the water cooling, performing tempering treatment at a        temperature of lower than the highest heating temperature Ac₁        point.

A steel material capable of suppressing ductile crack initiation fromwelded heat affected zone and a base material that can suppress ductilecrack initiation from a stress concentration zone, such as a weld toe,and prevent collapse or break of steel structures even when the steelstructures greatly deform due to an earthquake or the like, for example,can be easily and stably mass-produced and industrially remarkableeffects are demonstrated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating a ductile crack initiation test method ofa welded heat affected zone.

FIG. 2 is a view illustrating influence of the area fraction of a hardphase and the average aspect ratio of ferrite on ductile crackinitiation of a 1400° C. simulated heat cycle material.

FIG. 3 is a view illustrating a ductile crack initiation test method ofa base material.

FIG. 4 is a view illustrating influence of the area fraction of a hardphase and the average aspect ratio of ferrite on ductile crackinitiation of a base material.

DETAILED DESCRIPTION

The chemical composition and the microstructure are specified. In thedescription of the chemical composition, % by mass is simply representedby % unless otherwise specified.

Chemical Composition C: 0.02 to 0.2%

C is an element having an action of increasing the strength of steeland, particularly, contributes to the generation of a hard phase. A Ccontent of 0.02% or more is required to obtain such an effect. Incontrast, when the C content exceeds 0.2%, the ductility or the bendingworkability are reduced and also the weldability decreases. Therefore,the C content is limited in the range of 0.02 to 0.2%. More preferably,the C content is 0.02 to 0.18%.

Si: 0.01 to 0.5%

Si acts as a deoxidizing agent and has an action of forming a solidsolution to increase the strength of steel. An Si content of 0.01% ormore is required to obtain such an effect. In contrast, when the Sicontent exceeds 0.5%, the toughness is reduced and also the weldabilityis reduced. Therefor, Si is limited in the range of 0.01 to 0.5%. Morepreferably, the Si content is 0.01 to 0.4%.

Mn: 0.1 to 2.5%

Mn has an action of increasing the strength of steel and also increasingthe toughness through an increase in hardenability. An Mn content of0.1% or more is required to obtain such an effect. In contrast, when theMn content exceeds 2.5%, the weldability is reduced. Therefore, Mn islimited in the range of 0.1 to 2.5%. More preferably, the content is 0.5to 2.0%.

P: 0.05% or lower

Since P causes degradation of toughness, the P content is preferablyreduced as much as possible, but the content up to 0.05% is permissible.Therefore, the P content is limited to 0.05% or lower. More preferably,the content is 0.04% or lower.

S: 0.05% or lower

Since S is present as an inclusion in steel and degrades the ductilityand the toughness, the S content is preferably reduced as much aspossible. However, the content up to 0.05% is permissible. Therefore,the S content is limited to 0.05% or lower. More preferably, the contentis 0.04% or lower.

Al: 0.1% or lower

Al is an element that acts as a deoxidizing agent and also contributesto pulverization of crystal grains. However, an excessive content of Alin a proportion exceeding 0.1% causes a reduction in toughness.Therefore, the Al content is limited to 0.1% or lower. More preferably,the content is 0.05% or lower.

N: 0.01% or lower

N is an element that increases the strength of steel by solid solutionstrengthening similarly as C. However, an excessive content of N causesa reduction in toughness. Therefore, the N content is limited to 0.01%or lower. More preferably, the content is 0.005% or lower.

The chemical compositions described above are basic chemicalcompositions but one or two or more elements selected from Cu: 0.01 to2%, Ni: 0.01 to 5%, Cr: 0.01 to 3%, Mo: 0.01 to 2%, Nb: 0.1% or lower,V: 0.1% or lower, Ti: 0.1% or lower, B: 0.01% or lower, Ca: 0.01% orlower, and REM: 0.1% or lower may be further contained according to thedesired properties.

Cu: 0.01 to 2%

Cu is an element that has an action of increasing the strength of steelthrough an increase in hardenability or solid solution. The content of0.01% or more is required to secure such an effect. In contrast, whenthe content exceeds 2%, the weldability decreases and also cracks arelikely to generate during manufacturing of steel materials. Therefore,when Cu is added, the content is in the range of 0.01 to 2%. Morepreferably, the content is 0.01 to 1%.

Ni: 0.01 to 5%

Ni is added as required, because Ni contributes to an increase in lowtemperature toughness, an increase in hardenability, and prevention ofhot ductility of Cu when Cu is contained. Such an effect is recognizedwhen Ni is contained in the proportion of 0.01% or more. However, theaddition of 5% or more causes a reduction in steel material cost andalso a reduction in weldability. Therefore, when Ni is added, thecontent is in the range of 0.01 to 5%. More preferably, the content is0.01 to 4.5%.

Cr: 0.01 to 3%

Cr is added as required to increase the strength of steel materialsthrough improvement of hardenability or an increase in temperingsoftening resistance. Such an effect is recognized when Cr is containedin the proportion of 0.01% or more. In contrast, the addition exceeding3% reduces weldability and toughness. Therefore, when Cr is added, thecontent is in the range of 0.01 to 3%. More preferably, the content isin the range of 0.01 to 2.5%.

Mo: 0.01 to 2%

Mo is added as required to increase the strength of steel materialsthrough improvement of hardenability or an increase in temperingsoftening resistance. Such an effect is recognized when Mo is containedin the proportion of 0.01% or more. In contrast, the addition exceeding2% reduces weldability or toughness. Therefore, when Mo is added, thecontent is in the range of 0.01 to 2%. More preferably, the content isin the range of 0.01 to 1%.

Nb: 0.1% or lower

Nb is an element that precipitates as a carbide or a carbonitride intempering and increases the strength of steel through precipitationstrengthening. Moreover, Nb also has an effect of pulverizing austenitegrains during rolling to increase toughness. The content of 0.001% ormore is preferable to obtain the effects. However, the content exceeding0.1% reduces toughness. Therefore, when Nb is added, the content is 0.1%or lower. More preferably, the content is 0.05% or lower.

V: 0.1% or lower

V is an element that precipitates as a carbide or a carbonitride intempering and increases the strength of steel through precipitationstrengthening. Moreover, V also has an effect of pulverizing austenitegrains during rolling to increase toughness. The content of 0.001% ormore is preferable to obtain the effects. However, the content exceeding0.1% reduces toughness. Therefore, when Nb is added, the content is 0.1%or lower. More preferably, the content is 0.05% or lower.

Ti: 0.1% or lower

Ti is added as required because Ti has an effect of pulverizingaustenite in a welded heat affected zone to increase toughness. Thecontent of 0.001% or more is preferable to obtain the effect. However,the addition exceeding 0.1% reduces toughness and also causes a suddenrise of steel material cost. Therefore, when Ti is added, the content is0.1% or lower. More preferably, the content is 0.05% or lower.

B: 0.01% or lower

B is added as required because B has an effect of increasinghardenability and increasing the strength of steel with a small contentthereof. The content is preferably 0.0001% or more to obtain the effect.However, the addition exceeding 0.01% reduces weldability. Therefore,when B is added, the content is 0.01% or lower. More preferably, thecontent is 0.005% or lower.

Ca: 0.01% or lower

Ca is added as required because Ca increases the base material toughnessby controlling the shape of a CaS inclusion and further increase thetoughness of a welded heat affected zone. The content of 0.0001% or moreis preferable to obtain the effects. However, the addition exceeding0.01% reduces toughness due to an increase in the amount of the CaSinclusion. Therefore, when Ca is added, the content is 0.01% or lower.More preferably, the content is 0.009% or lower.

REM: 0.1% or lower

REM is an element that increases the toughness of a welded heat affectedzone and is added as required. The content is preferably 0.0001% or moreto obtain the effect. However, the addition exceeding 0.1% causes areduction in toughness. Therefore, when REM is added, the content is0.1% or lower. More preferably, the content is 0.05% or lower.

REM is a general term of Y, Ce and the like that are rare earth elementsand the addition amount as used herein refers to the total amount ofthese rare earth elements.

Microstructure

The steel material has a microstructure in which the structure at the ¼position of the plate thickness contains ferrite and a hard phase, thearea fraction of the hard phase is 50 to 90%, and the average aspectratio of the ferrite grain size is 1.5 or more. When the area fractionof the hard phase is lower than 50% and exceeds 90% or the aspect ratioof the ferrite grain size is lower than 1.5, there is a possibility thatductile crack initiation occurs.

The upper limit of the average aspect ratio of the ferrite grain sizedoes not need to particularly specify and is 5 or lower in view of thecapability and the like of a rolling mill. The area fraction of the hardphase is more preferably 52 to 90% and the average aspect ratio of theferrite grain size is more preferably 1.6 or more. The average aspectratio is more preferably 1.7 or more.

In a two phase mixed structure containing ferrite and a hard phase, theyield ratio (or Y/T ratio) of a base material decreases, and the strainconcentration in a stress concentration zone is eased even in the basematerial as it is or even after a simulated heat cycle of simulating thewelded heat affected zone. Such an effect is not obtained in the case ofa single phase of ferrite or a single phase of a hard phase.

In the steel material, the structure of the surface of a steel plate (1mm position from the plate surface) contains ferrite and a hard phase,in which the area fraction of the ferrite exceeds 40% and is morepreferably 50% or more. The average aspect ratio of the ferrite grainsize exceeds 2. When the area fraction of the ferrite is lower than 40%or the average aspect ratio of the ferrite grain size is 2 or lower, theresistance of ductile crack initiation in a welded heat affected zone ispoor.

The hard phase is bainite, martensite, or a bainite/martensite mixedstructure and contains 5% or lower, in terms of area fraction, of anisland martensite (M-A constituent) (MA).

FIG. 2 illustrates the results of examining the resistance of ductilecrack initiation using a simulated heat cycle specimen of a welded zone(highest heating temperature of 1400° C.). As illustrated in FIG. 2,when the area fraction of the hard phase of the base material is 50 to90% and the average aspect ratio of the ferrite thereof is 1.5 or more,ductile crack initiation is not observed also after the simulated heatcycle.

The results illustrated in FIG. 2 were obtained by specimens of 12 mmthickness (=plate thickness direction)×12 mm width×200 length from the ¼center of the plate thickness (½ center of the plate thickness in thecase of a plate thickness of 25 mm or lower) from the steel materialsobtained by producing steel having a composition in our range by variousmanufacturing methods and changing the microstructure, and then giving asimulated heat cycle (time for reaching the highest heating temperature:6 s, cooling rate from the highest heating temperature to roomtemperature: 40° C./s) of a welded zone thereto by a Gleeble tester toobtain sample materials.

FIG. 1 illustrates the specimen shape and the test conditions. Thesample material (specimen 1), to which the simulated heat cycle wasgiven, in which a single through-thickness edge notch is introduced withthe length of 3 mm in the plate thickness direction into the center of asimulated heat cycle zone 2 of the sample material (specimen 1) wasfixed with clamps 5, then a tensile load (arrow 6) was applied to 0.6 mmin terms of displacement of a clip gage 3 between knife-edges 4 that arescrewed, the load was removed, and then the specimen was ground to thecentral zone and mirror polished. Then, the presence of crack initiationat the notch tip was evaluated. The case where the ductile crack fromthe notch bottom was 50 μm or more was defined as crack initiation.

It is considered that the results illustrated in FIG. 2 are obtained dueto the fact that the yield ratio (or Y/T ratio) (0.2% proofstress/tensile strength) decreased also in the structure after thesimulated heat cycle and the degree of distortion concentration at thenotch tip zone decreased by the use of the base material having acomplex structure of ferrite and a hard phase.

Such outstanding characteristics were observed in common also in a basematerial to which the simulated heat cycle was not given.

More specifically, FIG. 4 illustrates the results of examining theinfluence of the microstructure of the base material exerted on theresistance of ductile crack initiation. As illustrated in FIG. 4, whenthe area fraction of the hard phase of the base material is 50 to 90%and the average aspect ratio of the ferrite is 1.5 or more, ductilecrack initiation is not accepted.

The results of the base material illustrated in FIG. 4 were obtained byspecimens of 12 mm thickness (=plate thickness direction)×12 mmwidth×200 length from the ¼ center of the plate thickness (½ center ofthe plate thickness in the case of a plate thickness of 25 mm or lower)from steel materials obtained by producing steel having a composition inour range by various manufacturing methods and changing themicrostructure.

FIG. 3 illustrates the specimen shape and the test conditions. Thesample material (specimen 1) in which a single through-thickness edgenotch is introduced into the center was fixed with clamps 5, then atensile load (arrow 6) was applied to 0.8 mm in terms of displacement ofa clip gage 3 between knife-edges 4 that are screwed, the load wasremoved, and then the specimen was ground to the central zone and mirrorpolished. Then, the presence of crack initiation at the notch tip wasevaluated. The case where the ductile crack from the notch bottom was 50μm or more was defined as crack initiation.

We believe that the results illustrated in FIG. 4 are obtained due tothe fact that the yield ratio (or Y/T ratio) (0.2% proof stress/tensilestrength) decreased and the degree of distortion concentration at thenotch tip zone decreased by the use of a base material having a complexstructure of ferrite and a hard phase.

Moreover, it is also considered to be one of the factors that the slipplane greatly leaned to the crack initiation direction in the basematerial as it is and also after the simulated heat cycle by increasingthe average aspect ratio of the ferrite, i.e., the development of thespecific aggregate structure. The aspect ratio refers to the ferritegrain size in the rolling direction (major axis)/the ferrite grain sizein the plate thickness direction (minor axis) in a cross sectionparallel to the rolling direction.

The same results as those of FIG. 2 were obtained also when the highestheating temperature of the simulated heat cycle was 760° C., 900° C.,and 1200° C.

The steel material is obtained by successively subjecting the steelmaterial of the above-described chemical compositions to a hot rollingprocess, a water cooling process, or further a tempering process.

The hot rolling includes reheating to 1000° C. or more and performingrolling in such a manner that the rolling reduction rate in atemperature range of 900° C. or more is 50% or more and the rollingfinish temperature becomes Ar₃ to Ar₃-50° C. A more preferable rollingfinish temperature is lower than Ar₃ to Ar₃-40° C. By setting therolling finish temperature in the invention range, processing strain (orresidual strain) can be added to ferrite generated during rolling tothereby increase the aspect ratio of the ferrite. When the reheatingtemperature is lower than 1000° C., hot rolling that gives a desiredcumulative rolling reduction rate cannot be performed to the steelmaterial.

When the cumulative rolling reduction rate at 900° C. or more is lowerthan 50%, desired strength and toughness cannot be secured. When therolling finish temperature exceeds Ar₃, the aspect ratio of ferrite doesnot reach 1.5 or more. When the rolling finish temperature is lower thanAr₃-50° C., the area fraction of the hard phase obtained by thesubsequent water cooling does not reach 50% or more.

In the water cooling process, the water cooling is started at Ar₃-10° C.to Ar₃-70° C. immediately after hot rolling, and then the water coolingis terminated at 500° C. or lower. When the water cooling starttemperature exceeds Ar₃-10° C., ferrite of lower than 10% in terms ofarea fraction (hard phase exceeding 90% in terms of area fraction)precipitates. When the water cooling start temperature is lower thanAr₃-70° C. or water cooling is not started immediately after (within 300seconds) hot rolling, ferrite exceeding 50% in terms of area fraction(hard phase not reaching 50% in terms of area fraction) or pearlite,which is not intended to precipitate, precipitates. Thus, desiredcharacteristics cannot be satisfied.

After performing the cooling, tempering treatment can be furtherperformed at a temperature of lower than the Ac₁ point. By performingtempering treatment, toughness and ductility increase, and desiredstrength and toughness can be achieved. When the tempering temperatureexceeds the Ac₁ point, a large amount of island martensite generates toreduce the toughness.

The Ar₃ point and the Ac₁ point can be calculated by the followingequation based on the content (% by mass) of each chemical composition:

Ar₃(° C.)=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo

Ac₁(° C.)=723−14Mn+22Si−14.4Ni+23.3Cr.

The disclosure will be described in more detail based on Examples.

EXAMPLES

Steel materials containing the chemical compositions shown in Table 1were subjected to hot rolling at the conditions shown in Table 2 tothereby obtain steel plates having a plate thickness of 12 to 100 mm.

The obtained steel plates were subjected to microstructure observation,a tensile test, a toughness test, a ductile crack initiation test aftera simulated heat cycle, and a ductile crack initiation test of basematerials. The test methods were performed as described in the followingitems (1) to (5).

(1) Microstructure Observation

From the obtained steel plates, specimens were extracted in the crosssection parallel to the rolling direction. Then, the specimens weremirror polished, and then etched with nital. Thereafter, themicrostructure at the ¼ position of the plate thickness and themicrostructure 1 mm below the surface were observed. The observation ofeach of the positions was performed with Field number: 20 fields ofview. The area fraction was determined by binarizing the ferrite and thehard phase and observing at a magnification of 200×. The average aspectratio of the ferrite was determined by determining the length in therolling direction and the length in the plate thickness direction ofeach ferrite present in the field of view at a magnification of 400×,determining the length in the rolling direction/the length in the platethickness direction, and then determining the average value thereof

(2) Tensile Test

From the obtained steel plates, full thickness JIS No. 5 specimens wereextracted so that the tensile direction was perpendicular to the rollingdirection of the steel plate according to the regulation of JIS Z 2201(1998). The tensile test was performed according to JIS Z 2241 (1998),and then the 0.2% proof (σ_(0.2)) and the tensile strength (TS) weredetermined to evaluate the static tensile properties.

(3) Toughness Test

From the obtained steel plates, V notch specimens were extracted so thatthe longitudinal direction was in parallel to the rolling directionaccording to the regulation of JIS Z 2242 (2005), and then theductile-brittle fracture transition temperature was determined toevaluate the toughness. The specimens were extracted in such a mannerthat the ¼ position of the plate thickness when the plate thickness was20 mm or more or the ½ position of the plate thickness when the platethickness was lower than 20 mm was the center.

(4) Ductile Crack Initiation Test after Simulated Heat Cycle

From the obtained steel plates, specimens of 12 mm thickness (=platethickness direction=t)×12 mm width and 200 mm in full length wereextracted at the ¼ center of the plate thickness (½ center of the platethickness when the plate thickness was 25 mm or lower). The specimenswere subjected to a simulated heat cycle of a welded heat affected zonein which the highest heating temperature was 760° C., 900° C., 1200° C.,and 1400° C. (time for reaching the highest heating temperature: 6 s,Cooling rate from the highest heating temperature to room temperature:40° C./s) using a Gleeble tester.

Thereafter, as illustrated in FIG. 1, a single through-thickness edgenotch was introduced with the length of 3 mm in the plate thicknessdirection into the center of the simulated heat cycle zone. The notchprocessing was carried out by electrical discharge machining, and thenotch tip radius was 0.1 mm.

In the test, a tensile load was applied while gripping the specimenswith both right and left ends thereof with a constraint length of 50 mm.During the test, the displacement between the knife-edges screwed nearthe notch was measured with the clip gage. A tensile load was applied to0.6 mm in terms of clip gage displacement, and then the load wasremoved. Thereafter, the specimen was ground to the width center andmirror polished. Then, the crack initiation state at the notch bottomwas analyzed under a microscope with a magnification of 50×. It wasdefined that the ductile crack initiation occurred when a ductile crackextended in the length of 50 μm or more from the notch bottom.

(5) Ductile Crack Initiation Test of Base Material

From the obtained steel plates, specimens of 12 mm thickness (=platethickness direction=t)×12 mm width and 200 mm in full length wereextracted at the ¼ center of the plate thickness (½ center of the platethickness when the plate thickness was 25 mm or lower).

To the obtained specimens, a single through-thickness edge notch wasintroduced with the length of 3 mm in the plate thickness direction intothe center of the specimens as illustrated in FIG. 3. The notchprocessing was carried out by electrical discharge machining, and thenotch tip radius was 0.1 mm.

In the test, a tensile load was applied while gripping the specimenswith both right and left ends thereof with a constraint length of 50 mm.During the test, the displacement between the knife-edges screwed nearthe notch was measured with the clip gage. A tensile load was applied to0.8 mm in terms of clip gage displacement, and then the load wasremoved. Thereafter, the test was ground to the width center and mirrorpolished. Then, the crack initiation state at the notch bottom wasanalyzed under a microscope with a magnification of 50×. It was definedthat the ductile crack initiation occurred when a ductile crack extendedin the length of 50 μm or more from the notch bottom.

With respect to the specimens that were subjected to the simulated heatcycle, the obtained experimental results are shown in Table 3. All ofthe steel plates of Nos. 1 to 10 produced using the chemicalcompositions and our manufacturing method have our structure. We foundthat the steel plates have excellent strength and toughness and haveexcellent resistance of ductile crack initiation of a welded heataffected zone.

In contrast, the steel plate (Steel type K*) of No. 11 in which the Ccontent does not satisfy the lower limit of our range has low tensilestrength. The steel plate (Steel type L*) of No. 12 in which the contentof each of C, P, and S exceeds the upper limit of our range has lowtoughness and has poor ductile crack initiation characteristics of awelded heat affected zone.

The steel plate of No. 13 in which the reheating temperature of slab islower than our and the cumulative rolling reduction rate at 900° C. ormore is outside our range has low toughness. In the steel plate of No.14 in which the rolling finish temperature and the water cooling starttemperature exceed our range, ferrite is not generated, ourmicrostructure is not obtained, and the resistance of ductile crackinitiation of a welded heat affected zone is poor.

In the steel plate of No. 15 in which the cooling start temperature islower than our range and the steel plate of No. 16 in which the watercooling stop temperature exceeds our range, the hard phase area fractionand the average aspect ratio of ferrite do not satisfy our values andboth the steel plates have low tensile strength and poor resistance ofductile crack initiation of welded heat affected zones. In the steelplate of No. 17 in which the tempering temperature exceeds our range,since a large amount of island martensite is generated, the toughness islow and the resistance of ductile crack initiation of a welded heataffected zone is poor.

The obtained experimental results of the base material are shown inTable 4. All of the steel plates of Nos. 18 to 27 produced using thechemical compositions and our manufacturing method have our structure.We found that the steel plates have excellent strength and toughness andhave excellent resistance of ductile crack initiation of a welded heataffected zone.

In contrast, the steel plate (Steel type W*) of No. 28 in which the Ccontent does not satisfy the lower limit of our range has low tensilestrength. The steel plate (Steel type X*) of No. 29 in which the contentof each of C, P, and S exceeds the upper limit of our range has lowtoughness. The steel plate of No. 30 in which the reheating temperatureof slab is lower than our range and the cumulative rolling reductionrate at 900° C. or more does not satisfy our range has low toughness.

In the steel plate of No. 31 in which the rolling finish temperature andthe water cooling start temperature exceed our range, ferrite is notgenerated, our microstructure is not obtained, and the resistance ofductile crack initiation is poor.

In the steel plate of No. 32 in which the cooling start temperature islower than our range and the steel plate of No. 33 in which the watercooling stop temperature exceeds our range, the hard phase area fractionand the average aspect ratio of ferrite do not satisfy our values andboth the steel plates have low tensile strength and poor resistance ofductile crack initiation. In the steel plate of No. 34 in which thetempering temperature exceeds our value, a large amount of islandmartensite (M-A constituent) is generated. Thus, the toughness is lowand the resistance of ductile crack initiation is poor.

REFERENCE SIGNS LIST

-   -   1. Specimen    -   2. Simulated heat cycle zone    -   3. Clip gage    -   4. Knife-edge    -   5. Clamp    -   6. Tensile load

TABLE 1 Steel Chemical composition (mass %) type C Si Mn P S Cu Ni Cr MoNb V A 0.14 0.33 1.59 0.005 0.002 — — — — — — B 0.06 0.11 1.96 0.0090.001 0.25 0.14 0.04 0.21 0.046 0.005 C 0.18 0.04 1.16 0.006 0.005 — — —— — 0.048 D 0.12 0.39 0.54 0.008 0.003 — — 0.22 0.35 — — E 0.03 0.260.52 0.003 0.005 — 4.49 — — 0.012 — F 0.09 0.32 1.32 0.042 0.007 0.981.33 — — 0.022 0.015 G 0.08 0.24 1.18 0.002 0.009 — — 2.48 0.98 0   0.018 H 0.11 0.18 1.22 0.001 0.004 — 0.52 0.16 0.21 0.045 0.033 I 0.050.32 1.38 0.002 0.043 — 2.43 — 0.25 0.033 0.019 J 0.09 0.25 1.44 0.0050.002 — — 0.08 0.11 0.022 0.038 K* 0.01* 0.21 1.56 0.004 0.003 — — — — —— L* 0.32* 0.18 0.55 0.193* 0.183* 0.25 0.15 0.23 0.14 — — M 0.15 0.321.58 0.006 0.001 — — — — — — N 0.06 0.12 1.95 0.008 0.004 0.22 0.15 0.020.21 0.045 0.004 O 0.19 0.02 1.15 0.005 0.005 — — — — — 0.047 P 0.110.38 0.52 0.007 0.003 — — 0.21 0.33 — — Q 0.03 0.25 0.51 0.003 0.004 —4.51 — — 0.018 — R 0.08 0.31 1.33 0.041 0.006 0.95 1.26 — — 0.025 0.011S 0.09 0.20 1.14 0.003 0.004 — — 2.45 0.97 — 0.009 T 0.12 0.16 1.250.004 0.003 — 0.55 0.15 0.22 0.043 0.032 U 0.05 0.27 1.37 0.001 0.042 —2.42 — 0.28 0.032 0.019 V 0.09 0.22 1.44 0.003 0.003 — — 0.07 0.15 0.0280.042 W* 0.01* 0.25 1.55 0.005 0.004 — — — — — — X* 0.31* 0.15 0.510.180* 0.173* 0.23 0.11 0.21 0.15 — — Chemical composition Ar₃ Ac₁ Steel(mass %) [° C.] [° C.] type Ti B Ca REM Al N Note (1) Note (2) A — — — —0.033 0.0048 739 729 B 0.011 0.0003 — — 0.032 0.0042 705 697 C — — — —0.044 0.0029 761 708 D — — — 0.097 0.029 0.0041 798 729 E 0.049 — — —0.018 0.0029 612 657 F — — 0.0089 0.031 0.022 0.0031 684 692 G — — — —0.025 0.0028 675 770 H 0.017 0.0046 — — 0.018 0.0035 731 706 I — —0.0033 0.044 0.022 0.0028 630 676 J 0.013 0.0011 — — 0.032 0.0041 757710 K* — — — — 0.028 0.0039 782 706 L* — — 0.0022 0.008 0.022 0.0038 739722 M — — — — 0.032 0.0048 737 729 N 0.012 0.0002 — — 0.031 0.0041 706697 O — — — — 0.048 0.0029 759 707 P — — — 0.098 0.028 0.0042 805 729 Q0.048 — — — 0.016 0.0028 612 656 R — — 0.0091 0.032 0.017 0.0041 691 693S — — — — 0.022 0.0032 677 769 T 0.018 0.0048 — — 0.011 0.0035 723 705 U— — 0.0032 0.048 0.023 0.0028 629 675 V 0.012 0.0015 — — 0.032 0.0041754 709 W* — — — — 0.031 0.0039 783 707 X* — — 0.0021 0.011 0.028 0.0037747 722 Note: The cells marked by * are outside our range and the steeltypes K, L, W, and X are comparative steels. Note (1): Ar3(° C.) =910-310 C—80 Mn—20 Cu—15 Cr—55 Ni—80 Mo Each alloy element amountindicates the content (%). Note (2): Ar1(° C.) = 723-14 Mn + 22 Si—14.4Ni + 23.3 Cr Each alloy element amount indicates the content (%).

TABLE 2 Cumulative rolling Water Water Slab reduction Rolling coolingcooling Plate reheating rate at finish start stop Tempering Steelthickness temperature 900° C. or temperature temperature temperaturetemperature No. type (mm) [° C.] more [%] [° C.] [° C.] [° C.] [° C.] 1A 14 1160 87 702 683 431 — 2 B 22 1190 75 679 665 378 — 3 C 12 1210 92731 687 298 — 4 D 100 1150 56 782 779 421 — 5 E 75 1240 62 583 578  72620 6 F 35 1190 73 672 634 388 — 7 G 24 1150 81 641 623  28 580 8 H 681240 55 701 687 426 — 9 I 34 1170 72 607 598 426 — 10 J 18 1160 82 718704 388 — 11 K* 22 1120 78 748 726 315 — 12 L* 45 1180 68 725 706 248 —13 C 73  970*  34* 741 732  42 650 14 A 14 1160 87  785*  777* 413 — 15B 28 1230 72 695  600* 388 — 16 J 19 1240 78 712 699  638* — 17 D 751090 64 768 749  62  760* 18 M 15 1150 86 695 681 401 — 19 N 20 1180 76686 664 308 — 20 O 12 1200 91 721 697 498 — 21 P 100 1130 55 792 777 418— 22 Q 75 1250 61 602 589 25 600 23 R 35 1200 72 668 643 418 — 24 S 251160 73 647 625  72 500 25 T 72 1250 58 708 697 457 — 26 U 37 1170 71618 605 412 — 27 V 15 1150 83 723 703 378 — 28 W* 25 1100 84 758 748 258— 29 X* 48 1200 69 721 710 243 — 30 O 75  930*  33* 737 717  23 600 31 M15 1150 88  755*  747* 428 — 32 N 25 1220 71 679  605* 352 — 33 V 181250 77 715 703  658* — 34 P 77 1080 62 776 748 245  750* Note: Thesteel types marked by * are outside our range.

TABLE 3 Microstructure of Microstructure 1 mm ¼ Plate thickness/4 belowthe surface Hard Hard phase Hard Ferrite phase Ferrite Ferrite struc-phase average struc- phase average Ductile crack initiation turefraction aspect ture fraction aspect σ0.2 TS vTrs characteristics Note(2) Classi- No. Note (1) [%] ratio Note (1) [%] ratio [MPa] [MPa] [° C.]760° C. 900° C. 1200° C. 1400° C. fication 1 B 59 1.9 B 55 2.4 428 548−57 ∘ ∘ ∘ ∘ Example 2 B 75 2.2 B 68 3.1 563 728 −105  ∘ ∘ ∘ ∘ Example 3B, M 54 2.3 B, M 77 4.8 521 689 −33 ∘ ∘ ∘ ∘ Example 4 B 64 1.6 B 48 2.2408 521 −29 ∘ ∘ ∘ ∘ Example 5 TB 90 1.7 TM 41 2.6 555 667 −98 ∘ ∘ ∘ ∘Example 6 B 62 1.8 B 59 2.3 473 621 −47 ∘ ∘ ∘ ∘ Example 7 TM 55 2.1 TM72 2.4 481 582 −92 ∘ ∘ ∘ ∘ Example 8 B 83 1.8 B 42 2.1 529 683 −64 ∘ ∘ ∘∘ Example 9 B 87 1.7 B 40 2.2 433 538 −41 ∘ ∘ ∘ ∘ Example 10 B 75 2.5 M58 3.3 428 548 −72 ∘ ∘ ∘ ∘ Example 11 B 73 2.2 B 53 3.0 325  421* −18 ∘∘ ∘ ∘ Comparative Example 12 B, M 72 1.7 B, M 43 2.3 677 991   15* ∘ x xx Comparative Example 13 TM 72 2.3 TM 40 2.5 521 609   8* ∘ ∘ ∘ ∘Comparative Example 14 B 100* — B  0* —* 548 678 −21 x x x x ComparativeExample 15 P  14*  1.1* P 87  1.4* 344  472* −11 x x x x ComparativeExample 16 P  21*  1.3* P 81  1.4* 388  488* −18 x x x x ComparativeExample 17 B, MA 63 1.8 M, MA 48 2.8 521 622   6* x x x x ComparativeExample Note: The cells marked by * are outside our range. Note (1): B:Bainite., M: Martensite, P: Pearlite, TB: Tempered bainite, TM: Temperedmartensite, MA: Island martensite Note (2): ∘: No ductile crackinitiation x: Ductile crack initiation

TABLE 4 Microstructure of Microstructure 1 mm ¼ Plate thickness belowthe surface Hard Hard Ferrite Hard Ferrite Ferrite Ductile crack phasephase average phase phase average initiation structure fraction aspectstructure fraction aspect σ0.2 TS vTrs characteristics No. Note (1) [%]ratio Note (1) [%] ratio [MPa] [MPa] [° C.] Note (2) Classification 18 B55 1.8 B 57 3.1 436 528 −48 ∘ Example 19 B 72 2.1 B 42 4.9 573 726 −121 ∘ Example 20 B, M 52 2.2 B, M 66 3.9 511 698 −21 ∘ Example 21 B 62 1.6 B48 2.2 359 515 −28 ∘ Example 22 TB 89 1.8 TM 41 4.1 552 628 −111  ∘Example 23 B 68 1.9 B 49 2.8 487 615 −35 ∘ Example 24 TM 59 2.0 TM 573.1 472 577 −98 ∘ Example 25 B 84 1.6 B 42 2.7 507 641 −63 ∘ Example 26B 88 1.7 B 43 2.9 402 513 −34 ∘ Example 27 B 77 2.4 B 53 3.3 425 538 −66∘ Example 28 B 74 2.1 M 58 3.7 368  411* −38 ∘ Comparative Example 29 B,M 78 1.8 M 55 2.2 687 983  10* ∘ Comparative Example 30 TM 69 2.4 TM 422.4 513 618   7* ∘ Comparative Example 31 B 100* — B  0* —:* 558 688 −18x Comparative Example 32 P  12*  1.2* P 89  1.4* 358  451* −13 xComparative Example 33 P  18*  1.4* P 83  1.4* 398  473* −21 xComparative Example 34 B, MA 68 1.7 M, MA 44 2.8 535 637   5* xComparative Example Note: The cells marked by * are outside our range.Note (1): B: Bainite., M: Martensite, P: Pearlite, TB: Tempered bainite,TM: Tempered martensite, MA: Island martensite Note (2): ∘: No ductilecrack initiation x: Ductile crack initiation

1. A steel material excellent in resistance of ductile crack initiationfrom welded heat affected zone and a base material, comprising: acomposition of C: 0.02 to 0.2%, Si: 0.01 to 0.5%, Mn: 0.5 to 2.5%, P:0.05% or lower, S: 0.05% or lower, Al: 0.1% or lower, and N: 0.01% orlower in terms of % by mass, and the balance Fe with inevitableimpurities, having a microstructure at ¼ position of plate thicknesscontaining ferrite and a hard phase, an area fraction of the hard phaseof 50 to 90%, and an average aspect ratio of the ferrite of 1.5 or more.2. The steel material according to claim 1, further comprising, in thechemical composition, one or more elements selected from the groupconsisting of Cu: 0.01 to 2%, Ni: 0.01 to 5%, Cr: 0.01 to 3%, Mo: 0.01to 2%, Nb: 0.1% or lower, V: 0.1% or lower, Ti: 0.1% or lower, B: 0.01%or lower, Ca: 0.01% or lower, and REM: 0.1% or lower in terms of % bymass.
 3. The steel material according to claim 1, wherein themicrostructure on a surface of a steel plate contains ferrite and a hardphase, the area fraction of the ferrite exceeds 40%, and the averageaspect ratio of the ferrite grain size exceeds
 2. 4. A method formanufacturing a steel material excellent in resistance of ductile crackinitiation from welded heat affected zone and a base materialcomprising: reheating a steel base material of claim 1 to 1000° C. ormore, rolling the same such that a rolling reduction rate in atemperature range of 900° C. or more is 50% or more and a rolling finishtemperature is Ar₃ point to Ar₃-50° C., starting water cooling atAr₃-10° C. to Ar₃-70° C., and terminating the water cooling at 500° C.or lower.
 5. The method according to claim 4 further comprising, afterthe water cooling, performing tempering treatment at a temperature lowerthan the highest heating temperature Ac₁ point.
 6. The steel materialaccording to claim 2, wherein the microstructure on a surface of a steelplate contains ferrite and a hard phase, the area fraction of theferrite exceeds 40%, and the average aspect ratio of the ferrite grainsize exceeds
 2. 7. A method for manufacturing a steel material excellentin resistance of ductile crack initiation from welded heat affected zoneand a base material comprising: reheating a steel base material ofclaims 2 to 1000° C. or more, rolling the same such that a rollingreduction rate in a temperature range of 900° C. or more is 50% or moreand a rolling finish temperature is Ar₃ point to Ar₃-50° C., startingwater cooling at Ar₃-10° C. to Ar₃-70° C., and terminating the watercooling at 500° C. or lower.